Nowadays, the application of the grounding resistance testers is increasingly widespread and important. At present, there are two main tools for measuring the grounding resistance: traditional grounding resistance meters; and a new type of clamped grounding resistance meter. The following is an analysis of the application of two grounding resistance measurement methods.
I Operating method
The ground wire of the grounding resistance meter must be released. More specifically, the grounding pole to be tested should be separated from the grounding system, and the voltage pole and the current pole must be driven into the soil at a prescribed distance as auxiliary electrodes for measurement, which is complicated and laborious.
The measurement of clamp grounding resistance meter belongs to a kind of non-contact methods. Only by clamping the ground wire to be tested with the bite of the clamp meter, the grounding resistance value can be read from the LCD screen of the clamp meter, which is quite simple and convenient.
II Accuracy of Measurement
The accuracy of the grounding resistance meter depends on the position pf the auxiliary electrodes and their relative position to the ground electrode. If the position of the auxiliary electrode is limited and cannot agree well with the calculated values, the so-called electrode arrangement error will occur. For the same ground electrode, different positions of auxiliary electrodes may cause a certain degree of dispersion of the measurement results and reduce the credibility of the the results.
The common clamped grounding resistance meter does not use an auxiliary electrode, so there is no electrode arrangement error. And even the test is repeated again and again, the result still maintain consistent.
III Testing Environment
The grounding resistance meter uses the voltage-current method. Two auxiliary grounding poles with a relative position must be driven into the ground. Therefore, there should be soil near the test point to meet the position requirements.
The clamp grounding resistance tester measures the loop resistance value, so the ground electrode to be tested must form a loop. The clamp meter cannot directly measure a single point grounding.
IV Other applications of clamp grounding resistance meter
In many environments, such as machine rooms in the basement or on the upper floors, lightning rods, elevators, gas stations, grounding bodies covered by concrete, and other grounding bodies that cannot be separated from the system, etc., it’s very difficult to test the grounding resistance with a conventional grounding resistance meter. Although they are all single-point grounded, if there are an available clamp meter on the scene, their grounding resistance can also be measured without disconnecting the ground electrodes from the system during the test. Fluke clamp grounding resistance meter can measure the ground faults that cannot be measured by conventional methods. For example, in a grounding system, the grounding resistance of their grounding poles is qualified, but after being used for too long, the connecting wire between the grounding poles and the lightning belt or between the grounding overhead lines may be disconnected after the contact a overlarge resistance. Therefore, although the grounding resistance of the pole is acceptable, but the whole grounding system is unqualified.
V Reasons of Large Metering Difference
If there is a big difference between the two metering results for the same grounding electrode, please pay attention to the following problems:
1.When a conventional grounding resistance meter is used, are the grounding down leads released? That is to confirm whether the grounding poles are disconnected from the grounding system). If they are not released, the resistance measured by the meter is the value of all grounding resistances in parallel of the grounding system, which is much smaller than the grounding resistance of the grounding electrodes, so the value is of no significance.
2. The sum of the grounding resistance of the grounding electrodesor the natural grounding electrodes and the grounding wire resistance is called the grounding resistance of the grounding device. Generally, traditional grounding resistance meter can only measured the grounding resistance of the grounding electrodes, the resistance of the grounding wires can not be measured. While the grounding resistance measured by the clamp meter is the sum of the grounding resistance of the grounding electrodes and the grounding wire resistance, which fully meets the standard
3. If the grounding system has only few grounding poles, there will be some errors in the measurement with the clamp meters, and generally the measured value would be larger.
In summary, there are certain conditions for both two measurement methods of grounding resistance. Each methods has its own advantages and disadvantages and cannot be replaced by each other. Therefore, users should choose a appropriate measurement method according to the system structure and environment of the ground electrodes in order to obtain a reliable measurement result.